1. Field of the Invention
The present invention relates to oil coating rollers for coating silicone oils as releasing agents on surfaces of fixing rollers in thermal fixing apparatuses for use in electrophotographic copying machines, laser printers, and the like.
2. Description of the Related Art
In electrophotographic processes represented by electrophotographic copying machines, toner images on copy paper are fixed by a thermal roller method. In the thermal roller method, a nip is formed by a fixing roller and a pressure roller, images formed by toner on copy paper are passed through the nip and are then heated and pressed, thereby fixing the toner by fusing. In the thermal roller method, in order to prevent toner from remaining on the fixing roller, the toner coming into direct contact therewith, so as to more preferably fix the toner images and decrease staining on a cleaner blade, a cleaner felt, and the like, a silicone oil having superior releasing properties is coated on the fixing roller.
In the case of monochrome images, the toner layer is thinner compared to the case of color images, and the amount of oil coated on a fixing roll can be further decreased concomitant with improvement of the toner itself. However, in the case of color images, since the toner layer is thick, the melting point thereof is low, and the viscosity thereof is high, the image deficiencies (toner offset) due to toner staining are easily generated, and as a result, in order to avoid the image deficiencies mentioned above, a larger amount of a silicone oil must be coated than that in the case of monochrome images.
As methods for coating silicone oil, there may be mentioned a method for coating a silicone oil, which is impregnated in a webbing, or a felt, on a surface of a fixing roller, and a method for coating a silicone oil, which is supplied by an oil coating roller, directly on a surface of a fixing roller. However, when a large amount of silicone oil is necessary as is the case with full-color images, a method for coating silicone oil is preferably performed by using an oil coating roller rather than that performed by using a webbing or a felt.
Conventionally, a topmost exterior layer of the oil coating roller is formed of a solid or a spongy silicone rubber layer. The spongy (foamed) silicone rubber is generally provided with a hollow member or the like therein for supplying a silicone oil, and the silicone oil is leached out on the spongy surface and is then coated, which is disclosed in Japanese Examined Patent Application Publications No. 5-60105, 7-15606, and the like. In the case of the solid (not spongy but solid form) silicone rubber layer, a silicone oil is transferred to an oil coating roller, which is scooped from a silicone oil bath, and it is then coated on a surface of a fixing roller by directly contacting the oil coating roller with the surface of the fixing roller. In addition, an oil-coating roller is disclosed in Japanese Unexamined Patent Application Publication No. 10-228205, in which the oil coating roller has the topmost exterior layer composed of a solid silicone rubber layer and a spongy silicone rubber layer provided therein.
In the method described above for coating silicone oil on the surface of the fixing roller by leaching the oil out on the spongy surface of the silicone rubber layer, toner adheres to the spongy surface in long-term use, oil supply is stopped at locations at which the toner adheres, and coating defects of the oil on the fixing roller occur, whereby there may be a problem in some cases in that abnormal images are produced. Accordingly, when an amount of toner offset is relatively large as is the case in which full-color images are fixed, particularly in order to perform stable and consistent oil coating over long periods of time, a method using a solid silicone rubber layer is preferably employed.
In addition, in the method for coating oil using the oil coating roller as described above, as a means for controlling the amount of silicone oil on the oil coating roller so as to be uniform, a method is generally used in which an oil control blade is provided which is urged toward the surface of the oil coating roller so as to maintain a constant pressure thereto by a pressure spring.
However, in conventional oil coating rollers in which the topmost exterior layers are composed of a solid silicone rubber layer, there are problems, which are of concern, as described below.
As a method for improving heat stability and mechanical strength of silicone rubber, a method is well known in which an inorganic filler, such as synthetic hydrated silica or silicate anhydride, is contained in the silicone rubber. However, since a large amount of synthetic hydrated silica, such as 20 to 50 parts by weight thereof to 100 parts by weight of the silicone rubber, must be added to obtain an effect of reinforcement, releasing properties of the silicone rubber may be degraded due to the addition of a large amount of synthetic hydrated silica. Consequently, an amount of offset toner transferred from the surface of the fixing roller is excessively increased, irregular and excessive coating of the oil on the oil coating roller thereby occur, and hence, excessive and irregular coating of the oil occur on the fixing roller, and ultimately, on a recording medium. As a result, it has been of concern that oil sagging, oil staining, oil stripes, and the like may be produced on images. In particular, the phenomena mentioned above may apparently be generated when an OHP sheet is fixed which will not absorb oil.
In addition, in the method for controlling an amount of silicone oil coated on the surface of the fixing roller by contacting an edge portion of the oil control blade urged toward the oil coating roller by a spring or the like, and at the same time, for smoothly and uniformly coating the oil on the oil coating roller, a uniform and desired amount of coating can be obtained at the beginning. However, as operation time passes, the edge portion of the oil control blade and the surface of the oil coating roller abrade each other, and abrasion of the edge portion thereby proceeds, whereby it may be difficult to maintain a stable coating amount of the oil in some cases after long operation time.
That is, in the case in which a coating amount of the oil is indicated by a when the oil control blade is in the initial state, and in the case in which a coating amount of the oil is indicated by b when the abrasion of the oil control blade proceeds, a  less than  less than b may occur in some cases. The reason for this is believed to be that the amount of the oil passing through is increased due to the progress in the abrasion of the edge portion of the oil control blade. Consequently, the amount of the oil transferred on the surface of the oil coating roller is excessively increased, and excessive coating on the fixing roller, and ultimately, on the recording media occurs, resulting in oil sagging, oil staining, and oil stripes. As a result, image quality is significantly degraded, and in particular, when images are fixed on an OHP sheet, sticking and degradation of tactile sensation due to excessive oil may occur in some cases.
The excessive coating amount of the oil not only increases consumption of the silicone oil but may also create problems in some cases, for example when double-sided copy is produced, paper supply defects (paper jams) due to slipping of the recording media when refed and cleaning defects, conveying defects, double images, and the like due to adhesion of excessive oil on the recording media on a photosensitive drum, a conveyer roller in a feed passage for conveying the recording media, and a conveyer belt.
On the other hand, in consideration of the increase in the coating amount of the oil due to the abrasion of the edge portion, when the oil control blade is in the initial state, the coating amount of the oil may be set to be less beforehand. In this case, the problems described above can be solved; however, new problems are of concern in which image qualities are significantly degraded. They are, for example, degradation of toner releasing properties due to insufficient coating amount of oil, decrease in life of the fixing roller, and increase in image gloss.
The increase in the coating amount of the oil of the oil coating device concomitant with long-term use is caused by the abrasion of the edge portion of the oil control blade. The abrasion of the edge portion occurs because a hard inorganic filler, which is contained in the silicone rubber layer forming the topmost exterior layer of the oil coating roller, is exposed on the surface thereof.
That is, in order to improve heat stability and mechanical strength, the topmost exterior layer composed of the silicone rubber layer of a conventional oil coating roller contains a large amount of a hard inorganic filler having relatively large diameters, and the inorganic filler is exposed on the surface of the topmost exterior layer. The exposure of the inorganic filler is more significantly observed when the oil coating roller is polished so as to obtain a desired diameter thereof. In addition, since the edge portion of the oil control blade is abraded with the inorganic filler having relatively large diameters exposed on the surface of the topmost exterior layer, the edge portion composed of a soft elastic material may be gradually abraded in some cases. As a result, since the oil passes through at locations at which the oil control blade is abraded, an amount of the oil supplied to the oil coating roller may be gradually increased in some cases. The phenomenon described above is more significant when the content of the inorganic filler is increased, and when the particle diameter thereof is increased.
In addition, the fixing roller may also have the problems described below. That is, when the initial amount of the oil coated on the surface of the fixing roller is insufficient, there may be problems in that the amount of the offset toner is increased, and the life of the fixing roller is decreased. In contrast, when the initial coating amount of the oil exceeds the appropriate range thereof, there may be problems in that color reproducibility of fixed images is degraded, oil staining occurs on recording sheets, tactile sensation of OHP sheets is degraded, and the like. Accordingly, the initial coating amount of the oil must always be controlled in the appropriate range.
As has thus been described, in the oil coating device in which the amount of silicone oil is controlled by the oil control blade which is urged toward the oil coating roller by a spring so as to maintain a constant pressure thereto, as a means for controlling an amount of silicone oil on the oil coating roller, conventionally, a means for adjusting a contacting angle of the oil control blade or a means for adjusting a pressure thereof contacting the oil coating roller are used. However, further improvement thereof has been desired.
Accordingly, it is an object of the present invention to provide an oil coating roller which has superior releasing properties and will not abrade an edge portion of an oil control blade.
It is another object of the present invention to provide an oil coating device using the oil coating roller described above and a fixing apparatus.
In the oil coating roller according to the present invention having a topmost exterior layer composed of a silicone rubber layer comprising a silicone rubber, the tensile strength of the silicone rubber layer is 1.0 MPa or more, and the silicone rubber layer contains no inorganic filler or contains not more than 0.5 part by weight to 100 parts by weight of the silicone rubber.
In addition, in the oil coating roller according to the present invention having a topmost exterior layer composed of a silicone rubber layer comprising a silicone rubber, the tensile strength of the silicone rubber layer is 1.0 MPa or more, and the silicone rubber layer contains no inorganic filler or contains an inorganic filler having an average particle diameter of less than 10 xcexcm.
According to the oil coating roller of the present invention, since the tensile strength of the silicone rubber layer is 10 MPa or more, it is not necessary to use a large amount of the inorganic filler having large diameter which causes abrasion of the edge portion of the oil control blade. As a result, long-term stability of the oil coating device can be realized.
In addition, by controlling the surface roughness of the roller surface of the silicone rubber layer in the appropriate range, the initial coating amount of the oil can be controlled in the appropriate range and can be maintained therein.
Furthermore, the fixing apparatus comprising the oil coating device of the present invention can provide stable and superior fixed images over long periods of time.
Further objects, features, and advantages of the present invention will become apparent from the following description of the preferred embodiments with reference to the attached drawings.